Abstract

A Cerenkov signal is generated when energetic charged particles enter the core of an optical fiber. The Cerenkov intensity can be large enough to interfere with signals transmitted through the fiber. We determine the spectrum of the Cerenkov background signal generated in a poly(methyl methacrylate) optical fiber exposed to photon and electron therapeutic beams from a linear accelerator. This spectral measurement is relevant to discrimination of the signal from the background, as in scintillation dosimetry using optical fiber readouts. We find that the spectrum is approximated by the theoretical curve after correction for the wavelength dependent attenuation of the fiber. The spectrum does not depend significantly on the angle between the radiation beam and the axis of the fiber optic but is dependent on the depth in water at which the fiber is exposed to the beam.

Schematic diagram of the experimental setup used to measure the spectrum of Cerenkov light generated in a PMMA fiber as a function of angle θ between −90° and 90°. A 200mm length of PMMA fiber was irradiated in air with a 9MeV electron beam from a Varian 21iX linear accelerator.

Spectrum of Cerenkov light generated by a 9MeV electron beam in a 200mm length of 1mm diameter PMMA fiber after transmission through an additional 20m of fiber. The fiber was irradiated at angles of 30°, 45°, and 60° to the fiber axis. The signal at 90° was too small to be detected. Error bars are shown for the 45° case and are of a similar magnitude for the other two angles.

Experimental setup that was used to measure the spectrum of Cerenkov light generated by a 6MV photon beam in a 100mm length of 1mm diameter PMMA fiber. The light was transmitted through an additional length of 20m of PMMA fiber.

Spectrum of Cerenkov light generated by a 6MV photon field in a 100mm length of 1mm diameter PMMA fiber after transmission through 20m of fiber. The fiber was irradiated with a 10cm×10cm field. Error bars are shown for the 45° case at 15mm depth and are of a magnitude similar to the other cases.

Experimental setup that was used to increase the intensity of Cerenkov light generated in a 6MV photon beam to enable more accurate measurement of its spectrum. The spectrum of Cerenkov light generated in a 2m length of 1mm diameter PMMA fiber was measured on the surface of a Solid Water phantom and at a depth of 15mm and 110mm. The light generated was transmitted through three 20m lengths of paired PMMA fiber to a spectrometer (six parallel fibers in total).

Spectrum of Cerenkov light generated in a 2m length of 1mm diameter PMMA fiber after transmission through 20m of fiber. The fiber was irradiated with a 25cm×25cm6MV photon beam at 90° to the fiber axis. The spectra are normalized at a wavelength of 500nm. Error bars are shown for the measurements made on the surface and are of a similar magnitude to those at the depths of 15 and 110mm. The theoretical Cerenkov spectrum from Fig. 3 is shown after attenuation through a 10, 20, and 50m length of PMMA fiber.